Epstein-Barr virus (EBV) causes infectious mononucleosis and is associated with a number of cancers including Burkitt lymphoma, nasopharyngeal carcinoma, Hodgkin lymphoma, and post-transplant lymphoproliferative disease. EBV infects B cells and 95% of adults are infected. EBV glycoprotein gp42 is essential for entry of virus into B cells. EBV gp42 binds to the 1 chain of human leukocyte antigens, HLA-DQ, -DR, and DP, on B cells, and uses these molecules for infection. To investigate if certain HLA-DQ alleles are associated with EBV seronegativity, we recruited 3,300 healthy adult blood donors, identified 106 EBV-seronegative individuals, and randomly selected a control group of EBV-seropositive donors from the donor pool. A larger than expected proportion of EBV-seronegative subjects were HLA-DQ 1 *04/*05 and *06/*06, and to a lesser extent, *02/*03, compared with the control group, while a larger than expected portion of EBV-seropositive persons were HLA-DQ 1 *02/*02. We examined the ability of EBV gp42 to bind to different HLA-DQ molecules using human and mouse cells stably expressing these alleles. EBV gp42 bound less effectively to cells expressing HLA-DQ 1 *04/*05, *06/*06, or *03/*03 than to cells expressing HLA-DQ 1 *02/*02. These data are consistent with our observations of increased EBV seronegativity with DQ 1 *04/*05 or *06/*06 alleles. These findings emphasize the importance of a single genetic locus (HLA-DQ 1) to influence infectivity with EBV. Human cytomegalovirus (HCMV) infects over half of the human population and is the most common infectious cause of birth defects. The virus is the most important infection occurring in transplant recipients. The mechanism of how HCMV enters cells is controversial. We found that THY-1, a cell protein that is critical for the early stage of entry of HCMV into certain cell types, contributes to virus entry by macropinocytosis, a cell process whereby cells take up water and nutrients. Using electron microscopy in two cell lines that support HCMV infection in a THY-1-dependent manner, we found that HCMV enters these cells by macropinocytosis. THY-1 associated with HCMV virions on the cell surface and colocalized with virus inside structures required for macropinocytosis. Inhibitors of macropinocytosis and soluble THY-1 blocked HCMV infection in the cell lines. HCMV entry into the cells triggered increased influx of fluid, a marker of macropinocytosis, and this increased fluid uptake was reduced by inhibitors of macropinocytosis and by soluble THY-1. These results suggest that HCMV has adapted to utilize THY-1 to facilitate efficient entry into certain cell types by macropinocytosis. Further knowledge about the mechanism of HCMV entry into cells may facilitate the development of novel inhibitors of virus infection.